Game concept
Fortified Holding
The game also links it as Fortified Holdings.
From the game's encyclopedia
A Fortified Holding is a Holding with Buildings that increase its Fort Level. County Capitals are always Fortified.
In order to fully Occupy a County, all its Fortified Holdings must be Occupied.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Fortified Holding is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Fortified Holding is a Holding with Buildings that increase its Fort Level." The game also links it as Fortified Holdings. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.